1. Field of the Invention
Many types of intermetallic compounds are known for use as hydrogen storage materials. Of particular interest herein are hydrogen storage materials provided by ZrCr.sub.2 -type alloys of the general formula ZrCrFeT.sub.x wherein T=Mn, Fe, Co, Ni and Cu wherein x=0.1 to 1.0, and their hydrides which are characterized as having the C14 hexagonal crystal structure.
2. State of the Art
A material suitable for storage of hydrogen must satisfy many manding criteria. In addition to large storage capacity for hydrogen, a hydrogen storage material should absorb and desorb hydrogen quickly, preferably at a pressure near one atmosphere, and the material should show a minimum of hysteresis effects during a hydrogen absorption/desorption cycle.
Intermetallic compounds which have received much attention for use as hydrogen storage materials are provided by derivatives of ZrCr.sub.2 compounds, which compounds are characterized by MgZn.sub.2 hexagonal C14 crystal structure. It is well known that the ZrCr.sub.2 system is capable of absorbing copious quantities of hydrogen, but that hydrides formed from the ZrCr.sub.2 system are too stable to be of practical significance. In search of improved ZrCr.sub.2 type systems for hydrogen storage, alloys have been prepared which contain other elements substituted for all or a portion of the zirconium, but with the ZrCr.sub.2 stoichiometry maintained in the new alloy. For example, in Shaltiel et al., J. Less-Comm. Metals, 53 117-131 (1977), there are described changes in properties of stoichiometric Laves-phase ZrCr.sub.2 -based compounds by substitution of chromium with a 3d transition metal in accordance with the empirical formula Zr(Co.sub.x M.sub.1-x).sub.2 and Zr(Fe.sub.x M.sub.1-x).sub.2 wherein M=V, Cr, Mn and x is between zero and one. For hyperstoichiometric ZrMn.sub.2 -based alloys Van Essen and Buschow, Van Essen et al., Mat. Res. Bull, 15, 2249-1155 (1980) and Pourarian et al., J. Phys. Chem., 85, 3105-3111, have shown that excess Mn in ZrMn.sub.2+x, x.ltoreq.1.8, raises the vapor pressure of the hydride by as much as 50-fold, which makes them more useful materials for hydrogen storage.